Examining indicators and methods for quantifying ozone exposure to vegetation

Abstract

The U.S. National Park Service uses W126 to assess critical levels of ozone (O3) exposure to sensitive plant species, whereas the scientific community considers the stomatal flux-based metric, phytotoxic O3 dose (POD), more relevant in foliar risk evaluation. This study found a decreasing trend of 0.15 ppm-hrs/yr (p = 0.049) in W126 at a Yellowstone National Park monitoring site during 1997–2021 and no trend at a Grand Teton National Park site over 2012–2021 but no trends at both sites in POD calculated following the United Nations Economic Commission for Europe Convention on Long-range Transboundary Air Pollution (CLRTAP) (2017). To evaluate the CLRTAP (2017) method, 2019 summer POD was calculated for comparison using the Big-leaf model and was found 20%–100% less than the CLRTAP (2017)-calculated leaf-level one already, let alone compared to the latter scaled up to canopy-level, owing to the much smaller stomatal O3 flux (SOF) estimates under most conditions. Further analysis revealed exceptions where larger SOF values (>0.02 mmol/m2/hr), obtained from using the Big-leaf model with measured sensible heat and water vapor flux included in input, were not captured by the CLRTAP (2017) method and the Big-leaf model with input of regular meteorological variables only. Those large SOF values resulted under cooler, less windy, more humid, and cloudier conditions, when the approximate water vapor concentration difference between the ambient and leaf saturation level was found <1 mol/m3. A multivariate function was developed, through machine learning, by regressing SOF on air temperature, wind speed, relative humidity, phytosynthetically active radiation, and O3 concentration. Its testing and evaluation demonstrated improved estimates of the large variability in SOF. This new parameterization showed promising capability of assessing critical levels of O3 exposure in park management, and it could be further improved with long-term flux measurement data becoming available and evapotranspiration better represented.